1. Technical Field
The present invention relates to a specimen used to measure a current flowing through a composite material containing conductive fibers and a current measuring method for the specimen.
2. Related Art
In recent years, the application of composite materials such as carbon fiber reinforced plastic (CFRP) to structures has been spreading widely. For example, such composite materials have been applied in a variety of fields, for example, to fuselages of airplanes, frameworks of automobiles, and blades of windmills. On the other hand, the composite material contains conductive fibers such as carbon fibers, and may thus serve as an unintended current path in connection with the design of the structure when a short circuit or lightning strike occurs. For example, in airplanes, fuel may be ignited by a possible spark caused by a lightning stroke. Accordingly, clarifying the current distribution in the fuselage during lightning is important.
However, the composite material, particularly a composite material containing conductive fibers such as carbon fiber reinforced plastic (CFRP), contains carbon fibers, which are electrically conductive, and a resin, which is an insulator, and is thus easily affected by a fiber arrangement direction. Thus, the composite material involves a more complicated current path than metallic materials. A particularly complicated current path is involved in a carbon fiber laminate in which a plurality of layers with different fiber directions is laminated. This leads to the need for a technique that allows more accurate measurement of a current flowing through the composite material.
For example, conventional inventions that allow measurement of the behavior or magnitude of a current flowing through the composite material are disclosed in Japanese Unexamined Patent Application Publication (JP-A) No. 2013-050306 and Japanese Unexamined Patent Application Publication (JP-A) No. 2013-053858. The invention disclosed in JP-A No. 2013-050306 enables visualization of local discharge when a current is passed through a composite material with a fastener. Furthermore, the invention disclosed in JP-A No. 2013-053858 enables measurement of a current in a carbon fiber laminate in which a plurality of layers of fibers in different directions is laminated.
The invention disclosed in JP-A No. 2013-050306 enables visualization of local discharge. Furthermore, the current measurement in the invention disclosed in JP-A No. 2013-053858 enables the distribution of currents in an in-plane direction in the composite material to be determined. However, in these inventions, it is not possible to measure a current in an out-of-plane direction that crosses the surface of in a laminate composite material. Additionally, a propagation mechanism for a current in an out-of-plane direction in the laminate composite material has not been theoretically clarified.
Moreover, both JP-A No. 2013-050306 and JP-A No. 2013-053858 disclose an example of current measurement in a specimen with a fastener. However, both JP-A No. 2013-050306 and JP-A No. 2013-053858 involve a specimen that is a single laminate composite material and fail to disclose an example of current measurement intended to clarify a propagation mechanism for a current in a specimen of two laminate composite materials coupled together with a fastener or conditions for a preferred coupling area.
Measures allowing smooth flow of a lightening current resulting from a lightning stroke and charge of static electricity are taken for an airplane structure to which a laminate composite material (composite material) is applied. This prevents thermal destruction of the structure resulting from application of a current to the structure and ignition in a fuel tank caused by discharge. In particular, for a lightning resistance measure, the fastener between the composite materials is desirably a bolt that offers reduced contact resistance to a base material to allow electricity to flow smoothly. However, conventionally available fasteners are special products and expensive, resulting in having an impact on fuselage manufacturing costs.
On the other hand, less expensive fasteners are also available, but when such fasteners are used without any modification, the fasteners may cause discharge because these fasteners offer large contact resistance to the composite material. Thus, the fasteners need a separate discharge measure. The large contact resistance is assumed to be ascribed to a difference in diameter between the bolt and a machined hole, a difference in fastener configuration, or a difference in a state of contact between the fastener and the machined hole resulting from different machining conditions.